This invention relates to a medical diagnostic ultrasound system and method for imaging blood flow or tissue movement. In particular, clutter filtering for flow or movement imaging is provided.
Ultrasound systems image blood flow and tissue movement using correlation or Doppler techniques. The flow or movement is represented by one or more of various estimated parameters, such as energy, velocity and/or variance. Prior to estimation of these parameters, a clutter filter may suppress undesired signals, such as associated with reflections of ultrasonic energy from stationary or slowly moving tissue.
The clutter filter response is typically selected as a compromise between low flow or movement sensitivity, clutter suppression, and the number of taps for the clutter filter. A large number of taps allows filters with a sharper magnitude versus frequency rolloff, more efficiently isolating desired signals from undesired signals (i.e. better differentiation between flow or movement and clutter). The large number of taps allows for better sensitivity to low velocity flow or movement. The large number of taps also allows for filtering with a flat frequency passband, providing more desirable sensitivity over a larger range of velocities.
However, more line firings are required for more taps. A larger number of line may reduce the frame rate for imaging.
U.S. Pat. No. 5,249,578 to Karp et al. teaches processing data with different finite impulse response filters where the different filters are defined as having reversed coefficients. The magnitude versus frequency response of these two filters is the same. By using these two filters, a greater number of independent samples may reduce the variance of velocity estimation.